As shown in FIG. 9, a magneto-optical disk 43, serving as a magneto-optical recording medium, essentially comprises a disc-shaped transparent substrate 44 and a magnetic film 45 formed on the substrate 44.
When information is recorded on the magneto-optical disk 43 by magnetic field modulation, a light beam 41 of constant intensity is converged by an objective lens 42 and is irradiated as a light beam spot 47 on the magnetic film 45 through the substrate 44. Since the temperature of a portion of the magnetic film 45 whereon the light beam spot 47 is irradiated rises to a vicinity of the Curie temperature, the magnetic coercive force of the portion of the magnetic film 45 decreases. Here, a magnetic field is applied to the magnetic film 45 from a recording magnetic head 46, the magnetic field reversing in response to the information to be recorded. Accordingly, mutually reversed magnetization directions are formed (shown by arrows in FIG. 9) in the portions of the magnetic film 45 whereon the light beam spot 47 is successively irradiated. The information is thereby recorded.
On the other hand, when the information is to be reproduced, a light beam 41 having a constant intensity (which is weaker than the intensity during recording) is converged by the objective lens 42 and is irradiated as a light beam spot 47 on the magnetic film 45 through the substrate 44, in the same way as described above. Light is reflected by the magnetic film 45 and returns to the objective lens 42 where it is converged. Information is then reproduced by detecting a rotation of a plane of polarization of the reflected light which is converged by the objective lens 42.
When recording frequency is increased in the case where the information is recorded by magnetic field modulation, magnetic domains 49 serving as recording units and located on the magnetic film 45 become crescent shaped, as shown in FIG. 10. This was reported in the "Digests of the 13th Annual Conference on Magnetics in Japan 1989" (page 198). That is, in a case where a track 48 on the magneto-optical disk 43 moves leftwards (as shown by an arrow in FIG. 10) with respect to the light beam spot 47 due to a rotation of the magneto-optical disk 43, areas of the magnetic film 45 are successively heated up by the irradiation of the light beam spot 47 thereon and, subsequently, each of the areas begins to cool down progressively in the rightward direction. Along with the progressive fall in temperature in each of the areas, the magnetic coercive force in each of the areas also begins to progressively increase in the rightward direction. When the temperature falls below a point where the magnetic coercive force becomes greater than the magnetic field applied from the recording magnetic head 46, the magnetization directions in the areas which have cooled below this point can no longer align with a direction of the magnetic field. For this reason, information is recorded in each of the areas just before the areas cool below this point. Accordingly, the shape of each of the magnetic domains 49 conforms to the shape of the left-hand side of the light beam spot 47, i.e., a crescent shape which is convex in the leftwards direction, as shown in FIG. 10.
The length (from left to right) of each of the crescent-shaped magnetic domains 49 is smaller than the diameter of the light beam spot 47. As the recording frequency is increased, the length of each of the magnetic domains 49 decreases further. Consequently, it becomes possible to achieve a high recording density by increasing the recording frequency.
However, in the conventional configuration described above, in the case where the recording frequency is increased, it becomes difficult to reproduce information by detecting the rotation of the plane of polarization of the reflected light since a plurality of the magnetic domains 49 fall under the light beam spot 47.